Why is it important to monitor disk I/O?

We are graphing various system parameters using Cacti. One of our graphs shows hard drive reads and writes. A question came up: why do we need this graph?

1. You need to know your read/write ratio to optimize RAID and Cache settings.
2. It helps diagnose bottlenecks.
3. Helps when planning upgrades; maintenance windows; capacity planning; etc.

You only need it if you want to know how your disk susbsystem is performing, or you want to troubleshoot disk related performance problems, or you want to establish a baseline performance metric for the disk susbsystem. If none of those things are your objective, then you don't need it.

Your question is more likely to get quality answers phrased like this "Why would we want to graph HDD reads\writes?" as opposed to "Why do we need to graph HDD reads\writes?" because like I said, you don't need it... but you may want it...